I’m fully with Damian on this one.
If some big raw prop is about to smash a player and the first thing he’s thinking about is bracing his TVA then he has a big problem.
I think the best ‘core exercise’ that can be done before training is stretching the hip flexors! Tightened hip flexors from constant sitting tend to allow the body to ‘switch off the core’, stretching them eases the lordosis and encourages a better posture. Also in the gym Bulgarian split squats are a great exercise too.